1. Field of the Invention
The present invention relates to birefringent fiber grating sensor systems and methods. In particular, the present invention relates to a birefringent fiber grating sensor system having a fiber grating sensor formed of an optical fiber with a plurality of side holes. In another embodiment, the present invention relates to a detection circuit for a birefringent fiber grating sensor system.
2. Description of the Invention
Various types of fiber optic sensor systems are known. In such systems, a transducer mechanism is used that affects the properties of light in an optical fiber in a manner that can be detected and that is indicative of a sensed parameter.
One example of such a transducer mechanism is a fiber grating sensor formed of a Bragg fiber grating recorded in a birefringent fiber. A Bragg grating comprises a periodic or semi-periodic refractive index structure recorded of an optical fiber. In the refractive index structure, the effective index of refraction is varied at a given spatial period (defined by a grating constant .LAMBDA.) along the length of the optical fiber. At a particular wavelength of the light, the period of the refractive index structure corresponds to the wavelength of the light guided in the optical fiber. Consequently, a resonance condition which is thereby created causes the light to reflect backwards. This resonance condition is known as the Bragg condition, and is stated mathematically as follows: EQU .lambda..sub.B =2n.sub.eff.LAMBDA. (1)
where .lambda..sub.B is the wavelength of light that is reflected backwards, n.sub.eff is the effective index of refraction of the propagating mode, and .LAMBDA. is the grating constant or period of the Bragg grating. In practice, real world constraints prevent the Bragg grating from being reflective only at the infinitesimally narrow spectral region defined by the discrete wavelength .lambda..sub.B. Instead, the Bragg grating is reflective in a narrow-banded spectral region that is centered about the wavelength .lambda..sub.B. Mathematically, however, the narrow-banded spectral region can be modeled as occurring at the discrete wavelength .lambda..sub.B. This simplification works quite well, and is utilized throughout the discussion contained herein.
In a birefringent fiber, light of one polarization experiences a different effective index of refraction than light of an orthogonal polarization. Thus, with reference to Eq. (1), it is seen that a Bragg grating recorded in a birefringent fiber reflects light at two different wavelengths.
This arrangement has been used to implement a pressure sensor. When pressure is applied to the pressure sensor, there is a change in the birefringence of the optical fiber and therefore a change in the spectral separation between the two wavelengths that are reflected. The change in separation of the two wavelengths is proportional to the amount of pressure applied. Thus, by monitoring the separation between the two wavelengths that are reflected, an indication of the pressure applied to the pressure sensor is obtained.
Fiber optic sensors enjoy popularity because they have several advantages over other types of sensors. First, because fiber optic sensors are non-conductive, they are particularly safe in applications where spark-induced fires or explosions are a concern. Additionally, for the same reason, fiber optic sensors are generally immune to lightning strikes and other sources of electromagnetic pulses or interference. Finally, fiber optic sensors, and in particular fiber grating sensors, can be fabricated in the optical fiber itself. Such fiber optic sensors are particularly easy to combine in wavelength-division multiplexed and/or time-division multiplexed fashion by disposing the sensors at various locations along an optical fiber.
However, several difficulties have been encountered in conjunction with fiber optic sensors systems, and more specifically in conjunction with birefringent fiber grating sensor systems. First, existing birefringent sensor systems do not exhibit satisfactory sensitivity. The change in birefringence of a birefringent sensor is in part caused by the geometrical asymmetry that occurs when the optical fiber deforms under strain. However, being made of glass, optical fibers are relatively difficult to deform and sensitivity is thereby limited. Moreover, traditional birefringent sensor systems have used detection techniques with limited sensitivity. Thus, what is needed is a fiber grating sensor arrangement that is more sensitive to sensed parameters such as pressure.
Second, existing birefringent sensor systems utilize sensor arrangements that are of limited flexibility. For example, in the context of pressure sensors, many existing fiber grating sensor arrangements are unable to measure differential pressure. Additionally, many existing fiber grating sensor arrangements do not provide temperature compensation. Thus, what is also needed is a fiber grating sensor arrangement that exhibits improved flexibility.
Finally, existing birefringent sensor systems utilize bulky detection systems that are implemented with free space (non-solid state) optics. The detection system is used to determine the spectral separation of the two reflections. Existing detection systems employ high resolution spectrometer techniques, such as performing a Fourier analysis on the coherence function of the reflected light to determine the spectral characteristics of the reflected light. However, in order to obtain high resolution coherence measurements, existing detection systems utilize an interferometer implemented with free space (non-solid state) optics. Such interferometers tend to be quite bulky and must operate in a low vibration (preferably, vibration-free) environment. Thus, what is needed is a detection system that can be implemented entirely in solid state optics/electronics but that exhibits the same sensitivity as more bulky, free space optics-based detection systems.